The invention relates to a technology that is effective in its being applied to a system and a method for reducing influence of cross-talk occurred in the case of wavelength-division-multiplexing a digital signal and an analog signal.
With the spread of the Internet, actualization of a large-capacity transmission service using optical fibers is underway. One of this type of large-capacity transmission services is a service in which multi-channel video signals are multiplexed by optical wavelength division multiplexing technology and thus distributed. A system utilized for this transmission service is established by the International Standardization s ITU (International Telecommunication Union) (G983.3) and is scheduled to be introduced into general households.
FIG. 8 shows a conventional example of an optical transmission system that multiplexes an analog optical signal and a digital optical signal by wavelength division multiplexing, and thus transmits the multiplexed signals. An optical transmission system P1 based on the prior art includes, an OLT (Optical Line Terminal) P2, a head end P3, a 1.55 μm (micrometer) optical transmitter P4, a star coupler P5, an ONU (Optical Network Unit) P6 and a user terminal P7. The OLT P2 includes a light transmitting/receiving unit P22, and a WDM (wavelength Division Multiplexing) optical coupler P23. A plurality of ONUs P6 and a plurality of user terminals P7 may be connected to one single star coupler P7. Downstream signals from the light transmitting/receiving unit P22 and the 1.55 μm optical transmitter P4 are wavelength-division-multiplexed by the WDM optical coupler P23, and transmitted through the star coupler P5 to the respective user terminals P7 via the respective ONUs P6.
FIG. 9 is a diagram showing an example of wavelengths of signals used in the optical transmission system P1 based on the prior art. FIG. 9 shows a case where the International s G983.3 are applied by way of a specific example. In the International s G983.3, 1.3 μm band (1.26 μm through 1.36 μm) is applied to upstream digital optical signals from a subscriber. Similarly, a 1.49 μm band (1.48 μm through 1.50 μm) is applied to downstream optical signals from the light transmitting/receiving unit P22, and a 1.55 μm band (1.55 μm through 1.56 μm) is applied to the downstream analog optical signals from the 1.55 μm optical transmitter P4, respectively. The downstream digital optical signal and the downstream analog optical signal are multiplexed by the WDM optical coupler P23 and thus transmitted to the side of subscriber (the side of the user terminal P7).
A video signal is given as one of specific examples of the downstream analog signals. An optical analog transmission system based on amplitude modulation multi-carriers (multiplexed carriers) using frequency division multiplexing (FDM), can be applied to transmission of the video signals. When this type of optical analog transmission system is applied, the head end P3 evenly allocates video channels at an interval of 6 MHz in several tens of MHz (megahertz) through 770 MHz. FIG. 10 is a diagram showing an example of the frequency division multiplexing of the video signals.
Note that a specific example of the prior art of this type of optical transmission system is a technology (refer to Patent documents 1, 2) for preventing, when the OLT P2 transmits the downstream signals to the plurality of user terminals P7, the downstream signals addressed to a certain user terminal P7 from being eavesdropped by other user terminal P7. According to this technology, for instance, the OLT P2 implements intrinsic scrambling upon the downstream signals addressed to the individual user terminals P7 and transmits these signals to the respective user terminals P7. Then, each user terminal P7 receiving this piece of data decodes the downstream signal by implementing descrambling by use of a key code associated with the user terminal P7 itself.
Further, technologies disclosed in Patent document 3 and Patent document 4 are given as examples of Raman depression method.
[Patent Document 1]
Japanese Patent Application Laid-Open Publication No. 06-188878
[Patent Document 2]
Japanese Patent Application Laid-Open Publication No. 07-193566
[Patent Document 3]
Japanese Patent Application Laid-Open Publication No. 2002-50985
[Patent Document 4]
International Publication Pamphlet No. 2002/035665